The present invention relates to a circuit arrangement provided with a switch mode power supply energized with pulsatory input current an output generated by high-frequency switching of main switch. The main switch is provided with a drive circuit comprising at least a drive switch connected to a return current line.
A circuit arrangement, where the switch mode power supply is constructed as a down-converter and connected to a public supply mains via a rectifier circuit is disclosed in U.S. Pat. No. 4,132,925. The switch mode power supply unit supplies a discharge lamp which acts as the electric load. It has become increasingly customary to supply switch mode power supplies through a preconditioner, for example, in order to meet requirements imposed on the current drawn from the supply mains (mains current requirements). The mains voltage is transformed to a DC voltage of 380 to 400 V in such a preconditioner. The preconditioner preferably forms part of the circuit arrangement which comprises the switch mode power supply.
Switch mode power supplies constructed in this way find their application as circuit arrangements for operating many kinds of electric loads, among them discharge lamps, but also, for example, small domestic appliances. The switch mode power supply may also be a commutator network, for example, a bridge circuit.
Bipolar transistors are used for the main switch and the drive switch in the known circuit arrangement. The use of bipolar transistors in applications in which a preconditioner as described above is used, however, has certain drawbacks, for example, the switching losses occurring at supply voltages of 380 V and higher become unacceptably high. Under such conditions the use of (MOS)FETs is more advantageous and practical. An important difference between a bipolar transistor and a (MOS)FET when used as a switching element is that a continuous base current is required to keep a bipolar transistor switched to the conducting state, whereas a voltage between gate and source of the order of 10 V must be maintained in the case of a (MOS)FET.
Switching the main switch into the conducting and non-conducting state in a switch mode power supply which acts as a supply source for an electric load is preferably done with a repetition frequency of 18 kHz or more, since there is a risk of acoustic interference at a lower repetition frequency. The use of a repetition frequency with such a high value requires that the switching process of the switch takes place very quickly, in a time interval of at most 1 .mu.s, but preferably much quicker. In the known circuit arrangement, however, this also contributes to comparatively high switching losses. This can be explained as follows.
In the known circuit arrangement, a high and a low voltage is alternately applied to the emitter of the bipolar transistor which acts as the drive switch. The drive circuit acting as level shifter contains a resistor as the current limiter. If switching of the switching means is to take place comparatively quickly, however, the resistance should be small, so that the switching process involves comparatively strong currents and accordingly comparatively great losses.
When (MOS)FET elements are used as the switching means, this has been counteracted in practice through the use of a pulse transformer in the drive circuit. This achieves a permanent electrical isolation between comparatively high voltage levels at the gate of the (MOS)FET acting as the main switch and a comparatively low drive voltage level. The use of a transformer, however, is disadvantageous since it cannot or cannot easily be integrated, is generally bulky and heavy, and moreover requires a separate drive if it is to operate satisfactorily.